Answer:
Who keeps the ocean clean?
Whoever makes it, tells it not. Whoever takes it, knows it not. Whoever knows it, wants it not. What am I?
It rows quickly with four oars but never comes out from under its own roof. What is it?
I am a ship that can be made to ride the greatest waves. I am not built by objects but built by minds.
What can travel around the world while staying in a corner?
Of these things - I have two. One for me - and one for you. And when you ask about the price, I simply smile and nod twice.